Cause
A thin tail means that the gecko is not getting enough food. This can be due to a lack of food availability (not feeding your colony enough), or bullying from other geckos who are higher in the colony pecking order.
Treatment
Make sure you are offering crested gecko diet 3 days per week and insects 2 days per week. If you are, the geckos are probably competing too much for food. Add a second or third dish of CGD (depending on the size of your colony) in a different location, and offer more insects per feeding.